Q: RELAX stops with an error about internal coordinates: keyword $intdef missing in file but I don't use internal coords at all!

A:If you have tried to automatically generate internal coordinates and 'iaut' failed, or if you have started with internal coords and changed later to another symmetry and told DEFINE that you do NOT want to use internal coordinates, it sometimes happens that the control file won't be changed. Check that you've switched to cartesian optimization: